The OTN standard according to ITU-T was architected and specified as an asynchronous optical transport standard with +/−20 ppm clock tolerance. At the point in time were the standard was created there was no need seen to use the OTU/ODU trails and paths as direct server for reference clock transport, i.e. to synchronize the OTUk/ODUk signals to a primary reference clock (PRC) or a synchronization supply unit (SSU). Extensive studies investigating the capability of transparently transporting synchronized services as SDH or synchronous Ethernet have been carried out during the development of ITU-T G.8251 Recommendation and have shown that the transport on base of the +/−20 ppm free running ODUk/OTUk sever clock presents good means for the OTN mapping and multiplexing to comply to the requirements defined for the synchronized services in terms of wander and jitter generation at OTN network element (NE) client ports. All this was based on statistically distributed free run OTN transport or HO-ODU (higher order optical-channel data unit) clocks in the +/−20 ppm window. Some influence of the used OTN clock tolerance on the short term wander/low frequency jitter generation due to multiplexing and mapping has been observed during those studies (what is also documented in G.8251 Appendix text).
At the present time, an enhancement in ITU-T is in discussion called SyncO—synchronous OTN—which shall allow to transport a reference clock along an OTU link. Another discussion is to transport PTP (IEEE 1588v2) over OTN.
When synchronizing the OTN transport or HO-ODU clocks, problems may arise when the frequencies of synchronous services transported in the ODU payload or LO-ODUs (lower order optical-channel data unit) multiplexed into HO-ODU payload area will be near to integer fractions of the transport rates. This leads to systematic justification patterns with mostly stable justification states and with few infrequent extra frequency justification actions for infrequent phase adjustments. This happens for both mapping-schemes used for such client configurations, i.e. for asynchronous mapping procedure (AMP) as well as generic mapping procedure (GMP).
US Patent Application US 2012/0039609 A1 discloses a method and an apparatus for transporting client signals in an OTN. In one embodiment, the method includes: receiving a client signal; determining a quantity of n-bit data units of the client signal based on a clock of the client signal and a local clock; mapping the quantity of n-bit data units of the client signal to an overhead of a first Optical Channel Data Tributary Unit (ODTU) frame; mapping the n-bit data units of the client signal to a payload area of a second ODTU frame next to the first ODTU frame according to the quantity of n-bit data units mapped in the overhead of the first ODTU frame; mapping each n-bit data unit of the second ODTU frame to an Optical Channel Payload Unit-k Tributary Slot in an OPUk frame; and forming an Optical Channel Transport Unit-k frame including the OPUk frame for transmission.
US Patent Application US 2013/0058643 A1 discloses a data amount derivation apparatus. The data amount derivation apparatus includes: a first calculator configured to derive, for one series of parallelized mapping signals, amount of data in each frame period for a frame into which the parallelized mapping signals are mapped; and a second calculator configured to sum up amounts of data in N frame periods, where N is an integer, and to derive the resulting summation value as the amount of data to be mapped into the frame, each of the amounts of data in each of the frame periods being derived by the first calculator.
So, there is a need to improve the transmission of synchronous data, specifically reference clock signals in an optical transport network with regards to short term wander and low frequency jitter generation.